This invention relates to functionalized periodic mesoporous materials and to their synthesis and use.
Porous inorganic solids have found great utility as catalysts and separations media for industrial application. The openness of their microstructure allows molecules access to the relatively large surface areas of these materials that enhance their catalytic and sorptive activity. Until recently, porous materials were generally divided into three broad categories using the details of their microstructure as a basis for classification. These categories are the amorphous and paracrystalline supports, the crystalline molecular sieves and modified layered materials. The detailed differences in the microstructures of these materials manifest themselves as important differences in the catalytic and sorptive behavior of the materials, as well as in differences in various observable properties used to characterize them, such as their surface area, the sizes of pores and the variability in those sizes, the presence or absence of X-ray diffraction patterns and the details in such patterns, and the appearance of the materials when their microstructure is studied by transmission electron microscopy and electron diffraction methods.
Amorphous and paracrystalline materials represent an important class of porous inorganic solids that have been used for many years in industrial applications. Typical examples of these materials are the amorphous silicas commonly used in catalyst formulations and the paracrystalline transitional aluminas used as solid acid catalysts and petroleum reforming catalyst supports. The term “amorphous” is used here to indicate a material with no long range order and can be somewhat misleading, since almost all materials are ordered to some degree, at least on the local scale. An alternate term that has been used to describe these materials is “X-ray indifferent”. The microstructure of the silicas consists of 100–250 Angstrom particles of dense amorphous silica (Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, Vol. 20, John Wiley & Sons, New York, p. 766–781, 1982), with the porosity resulting from voids between the particles. Since there is no long range order in these materials, the pores tend be distributed over a rather large range. This lack of order also manifests itself in the X-ray diffraction pattern, which is usually featureless.
Paracrystalline materials, such as certain aluminas, also have a wide distribution of pore sizes, but tend to exhibit better defined X-ray diffraction patterns usually consisting of a few broad peaks. The microstructure of these materials consists of tiny crystalline regions of condensed alumina phases, with the porosity of the materials resulting from irregular voids between these regions (K. Wefers and Chanakya Misra, “Oxides and Hydroxides of Aluminum”, Technical Paper No. 19 Revised, Alcoa Research Laboratories, p. 54–59, 1987). Since, there is no long range order controlling the sizes of pores in the material, the variability in pore size is typically quite high. The sizes of pores in these materials fall into a regime called the mesoporous range which, for the purposes of this application, is from about 2 to about 50 nm.
In sharp contrast to these structurally ill-defined solids are materials whose pore size distribution is very narrow because it is controlled by the precisely repeating crystalline nature of the materials' microstructure. These materials are called “molecular sieves”, the most important examples of which are zeolites.
Zeolites, both natural and synthetic, have been demonstrated in the past to have catalytic properties for various types of hydrocarbon conversion. Certain zeolitic materials are ordered, porous crystalline aluminosilicates having a definite crystalline structure as determined by X-ray diffraction, within which there are a large number of smaller cavities which may be interconnected by a number of still smaller channels or pores. These cavities and pores are uniform in size within a specific zeolitic material. Since the dimensions of these pores are such as to accept for adsorption molecules of certain dimensions while rejecting those of larger dimensions, these materials are known as “molecular sieves” and are utilized in a variety of ways to take advantage of these properties.
Such molecular sieves, both natural and synthetic, include a wide variety of positive ion-containing crystalline silicates. These silicates can be described as a rigid three-dimensional framework of Periodic Table Group IVB element oxide, e.g. SiO4, and Periodic Table Group IIIB element oxide, e.g. AlO4, in which the tetrahedra are cross-linked by the sharing of oxygen atoms whereby the ratio of the total Group IIIB element, e.g. aluminum, and Group IVB element, e.g. silicon, atoms to oxygen atoms is 1:2. The electrovalence of the tetrahedra containing the Group IIIB element is balanced by the inclusion in the crystal of a cation, for example, an alkali metal or an alkaline earth metal cation. This can be expressed wherein the ratio of the Group IIIB element to the number of various cations, such as Ca/2, Sr/2, Na, K or Li, is equal to unity. One type of cation may be exchanged either entirely or partially with another type of cation utilizing ion exchange techniques in a conventional manner. By means of such cation exchange, it has been possible to vary the properties of a given silicate by suitable selection of the cation.
The precise crystalline microstructure of most zeolites manifests itself in a well-defined X-ray diffraction pattern that usually contains many sharp maxima and that serves to uniquely define the material. Similarly, the dimensions of pores in these materials are very regular, due to the precise repetition of the crystalline microstructure. All molecular sieves discovered to date have pore sizes in the microporous range, which is usually quoted as 0.2 to less than 2.0 nm, with the largest reported being about 1.3 nm.
Certain layered materials, which contain layers capable of being spaced apart with a swelling agent, may be pillared to provide materials having a large degree of porosity. Examples of such layered materials include clays which may be swollen with water, whereby the layers of the clay are spaced apart by water molecules. Other layered materials are not swellable with water, but may be swollen with certain organic swelling agents such as amines and quaternary ammonium compounds. Examples of such non-water swellable layered materials are described in U.S. Pat. No. 4,859,648 and include layered silicates, magadiite, kenyaite, trititanates and perovskites. Another example of a non-water swellable layered material, which can be swollen with certain organic swelling agents, is a vacancy-containing titanometallate material, as described in U.S. Pat. No. 4,831,006.
Once a layered material is swollen, the material may be pillared by interposing a thermally stable substance, such as silica, between the spaced apart layers. The aforementioned U.S. Pat. Nos. 4,831,006 and 4,859,648 describe methods for pillaring the non-water swellable layered materials described therein and are incorporated herein by reference for definition of pillaring and pillared materials. Other patents teaching pillaring of layered materials and the pillared products include U.S. Pat. Nos. 4,216,188; 4,248,739; 4,176,090; and 4,367,163; and European Patent Application 205,711.
The X-ray diffraction patterns of pillared layered materials can vary considerably, depending on the degree that swelling and pillaring disrupt the otherwise usually well-ordered layered microstructure. The regularity of the microstructure in some pillared layered materials is so badly disrupted that only one peak in the low angle region on the X-ray diffraction pattern is observed, at a d-spacing corresponding to the interlayer repeat in the pillared material. Less disrupted materials may show several peaks in this region that are generally orders of this fundamental repeat. X-ray reflections from the crystalline structure of the layers are also sometimes observed. The pore size distribution in these pillared layered materials is narrower than those in amorphous and paracrystalline materials but broader than that in crystalline framework materials.
More recently, a new class of porous materials has been discovered, see U.S. Pat. No. 5,102,643, and has been the subject of intensive scientific research. This class of new porous materials, referred to as the M41S materials, may be classified as periodic mesoporous materials and comprise an inorganic porous crystalline phase material having pores with a diameter of 1.5 to 30 nm, which is larger than known zeolite pore diameters. The pore size distribution is generally uniform and the pores are regularly arranged. The pore structure of such mesoporous materials is large enough to absorb large molecules and the pore wall structure can be as thin as about 1 nm. Further, such mesoporous materials are known to have large specific surface areas (about 1000 M2/g) and large pore volumes (about 1 cc/g). For these reasons, such the mesoporous materials enable reactive catalysts, adsorbents composed of a functional organic compound and other molecules to rapidly diffuse into the pores and are therefore, advantageous over zeolites, which have smaller pore sizes. Consequently, such mesoporous materials find potential high-speed catalytic reactions and as large capacity adsorbents.
One problem with existing periodic mesoporous materials is that the relative inactivity of the materials limits their utility in catalytic reactions. Various proposals have therefore been made to enhance their activity by functionalizing the materials.
For example, U.S. Pat. No. 5,145,816 discloses functionalization of periodic mesoporous materials by post-synthesis treatment with a composition comprising M′X′Y′n wherein M′ is selected from Periodic Table Groups IIA, IIIA, IVA, VA, VIA, VIIIA, IB, IIB, IIIB, IVB, VB and VIB; X′ is selected from halides, hydrides, alkoxides of 1 to about 6 carbon atoms, alkyl of C1-18, alkenyl of C1-18, aryl of C1-18, aryloxide of C1-18, sulfonates, nitrates and acetates; Y′ is selected from a group consisting of X′, amines, phosphines, sulfides, carbonyls and cyanos; and n=1–5. However, post-synthesis functionalization is often accompanied by substantial deceases in pore diameter and pore volume.
PCT Publication No. WO9834723 describes attaching organic groups onto the surface of the inorganic skeleton of periodic mesoporous materials, namely onto the inner surface of the pores, so as to impart selective adsorption ability and specific catalyst functions to the mesoporous substance. Such mesoporous materials are formed with organic groups bound as side chains suspended from the surface of the inorganic base skeleton. Consequently, the pore wall is basically composed of an inorganic skeleton with the organic groups projecting from the surface of the pore wall to form a layer composed of the organic groups.
In such a structure, the surface characteristics of the porous material are determined by the characteristics of the organic groups. As a result, such porous materials are restricted to adsorbing substances to which the organic groups have affinities. Further, the catalytic function or adsorption function derived from the inorganic skeleton can be masked, because the catalytically active sites or adsorption sites in the inorganic skeleton are covered by the organic groups. In addition, the thickness of the pore wall also may increase corresponding to the introduction of the organic group, thereby resulting in substantial decreases in pore diameter and pore volume of the molecular sieve. Further, such organic groups may release under high temperatures or in a catalytic reaction and adsorption process, thus leading to the loss of desirable surface properties and the contamination of the treated material by the released organic group.
In an attempt to alleviate the shortcomings of surface attachment of organic groups, U.S. Pat. No. 6,248,686, which is incorporated herein by reference, teaches incorporating an organic group into the skeleton of a mesoporous material. More specifically, the patent teaches incorporation of an organic group into the mesoporous skeleton such that the organic group is bound to at least two metal atoms in the skeleton. It is reported that the inclusion of the organic group into the skeleton of the mesoporous material confers the properties of the organic group on the mesoporous material without substantially reducing its pore diameter or pore volume. Among the organic groups disclosed in the '686 patent are alkylene groups, alkenylene groups, vinylene groups, alkynylene groups, phenylene groups, hydrocarbons containing phenylene groups, amido groups, amino groups, imino groups, mercapto groups, sulfone (═SO2) groups, carboxyl groups, ether groups and acyl groups.
According to the present invention, a new class of solid acid periodic mesoporous materials have been discovered in which the materials have bridging organic groups containing sulfonic acid moities incorporated into the framework. The resulting sulfonic acid functionalized mesoporous materials have rigid, accessible, reactive and uniformly distributed acid groups and exhibit chemical and physical properties suggesting potential utility in heterogeneous catalysis.